Mechanical forces are key regulators of cellular function for both normal growth and homeostasis of tissues, and in diseases including atherosclerosis, hypertension, osteoporosis and cancer. Understanding the mechanisms by which cells sense and respond to mechanical forces is therefore critical for understanding normal physiology and these disease states. Integrin-mediated adhesions provide the primary linkage through which cells both transmit and sense mechanical forces between the extracellular matrix (ECM) and the cytoskeletal networks within the cell. Current views of mechanosensing center on the concept of a molecular clutch that connects the immobile, bound integrins to rearward moving actin filaments. These connections are formed through load-bearing linker molecules such as talin. Thus, forces are sensed by an intrinsically dynamic assembly whose behavior is modified by mechanical forces. Understanding this system in molecular detail is the goal of the project. The Schwartz lab has developed molecular force sensors that allow spatial and temporal measurement of tension across specific proteins. I propose to combine a talin tension sensor with fluorescent speckle microscopy for visualization of actin filament assembly, movement and disassembly. This approach will allow simultaneous analysis at near-single molecule levels of the forces and dynamics within cell adhesions. This will be done while varying both cell-derived and externally applied forces. Determining how forces modulate the molecular dynamics within the adhesions will provide fundamental insights into the molecular mechanisms of mechanotransduction by integrin-mediated adhesions.